FIG. 1 is a diagram illustrating a wireless communications network 1000 that includes one or more wireless devices 110, such as a user equipment (UE), one or more radio access nodes 120, such as an eNodeB (eNB), and various network nodes 130. Examples of network nodes 130 may include a mobility management entity (MME), a switch or server (e.g., a mobile switching center (MSC)), a media gateway (MGW), a serving GPRS support node (SGSN), a serving gateway/packet data network gateway (S-GW/PDN GW), an IP multimedia system (IMS), a base station controller (BSC), a radio network controller (RNC), and/or other suitable nodes. Wireless device 110 communicates with radio access node 120 over a wireless interface. For example, wireless communication device transmits wireless signals to radio access node 120 and/or receives wireless signals from radio access node 120. The wireless signals contain voice traffic, data traffic, and control signals, for example. One or more network nodes 130 manage the establishment of communication sessions and various other functionalities for wireless device 110. The radio access node 120 and the network nodes 130 connect through interconnecting network 125, which refers to any interconnecting system capable of transmitting audio, video, signals, data, messages, or any combination of the preceding.
In certain situations, wireless device 110 may move from packet switched (PS) access, such as Evolved-Universal Terrestrial Radio Access Network (E-UTRAN), to circuit switched (CS) access, such as UTRAN or GSM EDGE Radio Access Network (GERAN). To continue the voice call/session when moving from PS access to CS access, single radio voice call continuity (SRVCC) technology can be used. FIGS. 2A-2B illustrate an example of a SRVCC message flow described in the 3GPP TS 23.216 specification. In particular, the figures illustrate the message flow for a wireless device 110 (UE) moving from E-UTRAN to GERAN. For a detailed description of each step in this figure, please refer to 3GPP TS 23.216.
From the perspective of the MSC server, the first message for SRVCC is the PS to CS request (step 5 in FIG. 2A). The PS to CS request includes a session transfer number for SRVCC (STN-SR) and an identification of the target, such as a target RNC ID or a target Cell ID. The STN-SR identifies an IMS node, such as an Access Transfer Control Function (ATCF) or a Service Centralization and Continuity Application Server (SCC-AS) that performs the bearer path update during SRVCC. Based on the data from the PS to CS Request, the MSC sets the bearer path to the target RNC/BSS. After the bearer path to target RNC/BSS is set up, the MSC initiates the session transfer (step 10 in FIG. 2A) and sends a PS to CS Response to the MME (step 13 in FIG. 2A).
As mentioned by 3GPP TS 23.216, in FIG. 2A, steps 11 and 12 are independent of step 13. This means that the time for the UE to tune to circuit switched access (Tcs) is most likely different than the time for the IMS side to finish the session transfer of the voice path (Tps) whereby the IMS connects the remote end with the bearer path towards the MSC server. A voice gap/interruption occurs when Tps and Tcs are different. The duration of the voice gap/interruption can be determined by calculating the difference between the two times (Tps−Tcs). In general, as the duration of the voice gap/interruption increases, the end user experience gets worse.
Several ideas have been considered to reduce this voice gap. As an example, Release 10 SRVCC specifications introduce the ATCF, which assumes that Tps is normally greater than Tcs if the session transfer is performed in the home network which may be different from the visiting network to which the MSC server belongs. The ATCF anchors the bearer for the IMS and is normally put in the visiting network. During SRVCC, the ATCF performs session transfer. Because the ATCF and the MSC server are both in the visiting network, the Tps may be reduced and hence the voice gap is reduced.
As another example, in European Patent Application EP 2343922 A1 entitled “Enhancement of single radio voice call continuity mobility,” the MSC server waits to send the handover command (PS to CS Response) until after it has received a report indicating that the media gateway under its control has received the first packet from the interworking agent.
As another example, in European Patent Specification EP 2244504 B1, entitled “Reduction of flow break in sr-vcc mobility,” the MSC server sets a delay timer value in the handover command (PS to CS response). When the UE receives the handover command with this timer, it delays the access transfer for the duration specified by the timer.
Returning to FIG. 2A, at step 14 the source MME sends a Handover command to the Source E-UTRAN, at step 15 the source E-UTRAN sends a HO from E-UTRAN command to the UE, and at step 16 the UE tunes to the GERAN. Continuing to FIG. 2B, steps 17-24 show the release of the PS side after HO to the CS side. For a detailed description of each step in this figure, please refer to 3GPP TS 23.216.